Chemical stripping process

ABSTRACT

Aluminide coatings are removed without attack of the substrate by immersion of coated articles in an aqueous liquid composed of a mixture of orthophosphoric acid, nitric acid and acetic and/or chloroacetic acid. For more rapid chemical stripping, the mixture of acids should be heated.

United States Patent 2,446,060 7/1948 Pray et a1.. 2,650,157

Inventor Joseph G. Lucas Trumbull, Conn.

Appi. No. 834,522

Filed June 18, 1969 Patented Sept. 21, 1971 Assignee Avco Corporation Stratiord, Conn.

CHEMICAL STRTPPING PROCESS 11 Claims, No Drawings 11.5. C1 134/3, 134/41, 252/79.4, 25 2 5; Int. Cl B081) 3/08, C23g 1/12 Fieid of Search 134/3, 41; 252/792, 79.4, 101, 142; 75/121 References Cited UNITED STATES PATENTS 8/1953 Cochran 2,678,875 5/1954 Spooner 252/79.2 X 3,008,812 11/1961 Spahn 252/142 X 3,041,227 6/1962 Jumer 252/101 X 3,119,726 1/1964 King et a1.. 252/101 X 3,202,612 8/1965 Neison 252/79.2 3,458,353 7/1969 Baidi 134/3 3,514,407 5/1970 Missei 134/41 X FOREIGN PATENTS 174,489 2/1922 Great Britain 252/101 474,228 9/1952 Italy 252/79.2

Primary Examiner-Joseph Scovronek Assistant Examiner-D. G. Miliman Att0rneys-Char1es M. Hogan, irwin P. Garfinkle and Lawrence 1. Field ABSTRACT: Aluminide coatings are removed without attack of the substrate by immersion of coated articles in an aqueous liquid composed of a mixture of orthophosphoric acid, nitric acid and acetic and/or chloroacetic acid. For more rapid chemical stripping, the mixture of acids should be heated.

CHEMICAL STRIPPING PROCESS This invention relates to the removal of aluminide coatings from nickelor cobalt-base alloy substrates.

One surface treatment to improve the life of rotors and other components used in gas turbine engines involves the coating of the nickel-base or cobalt-base superalloys of which the components are commonly fabricated. One known coating process involves covering the parts with aluminum or aluminum base alloys and then be subsequent treatment converting the coating to an aluminide of nickel or cobalt.

Suitable base materials for the turbine parts are described in an earlier patent issued on an invention made jointly be me, namely, US. Pat. No. 3,155,536, issued Nov. 3, 1964, and are known in the art as superalloys.

Later developments in the coating of such alloys are described in a copending patent application, U.S. Ser. No. 686,852 filed Nov. 30, 1967.

Other patents which describe the formation of aluminide coatings of Nior Co-base alloys and superalloys include US. Pat. Nos. 3,079,276 and 3,096,160 issued Feb. 26, 1963, and July 2, 1963, respectively.

in many instances, the aluminide coating is not suited to the intended application either because it is too thick or too thin, or because the aluminide coating is not uniform, or is incomplete on some area of the article to be protected.

The present invention is directed to the removal of such unsuitable coatings without any damage to the base material so that the turbine vane or bucket or other component can be salvaged whether it be for the purpose of recoating, machining and then recoating, or for the purpose of recovering the alloy by melting the alloy, uncontaminated by the coating. Because of the complicated shape of such parts, machining or other forms of mechanical removal of the aluminide coating is not feasible. Further, when the parts are hollow, a coating inside the bore is usually inaccessible to mechanical tools.

The present invention provides a chemical method for stripping the aluminide coating from such base materials, by use of a concentrated mixture of phosphoric, nitric and acetic acids.

One preferred composition comprises the three acids in the proportions 2: l :1 by volume, but the proportions can be varied over the following ranges (percent of volume):

Broad Preferred mm, (85% cone) -80 40-60 HNO (70% cone) 10-80 l530 Glacial Acetic (99% cone) 015-85 -30 From the above, it will be seen that the final mixture contains several percent of water.

Stripping of the aluminide coating from the base material is accomplished by simply immersing the coated article in a solution consisting of a mixture of the three acids formed by adding the nitric and acetic acids to the phosphoric acid and stirring the mixture.

The preferred formula No. 4 above (2:1: 1 gives a strip rate of approximately 0.001 inch per 10 minutes at 165F.

In order to avoid the evolution of unpleasant acetic acid vapors from the heated stripping solution, chloroacetic acid may be used in place of some or of all of the glacial acetic acid. The use of chloroacetic acid has been found to be particularly desirable when stripping coatings which contain relatively high percentages of chromium in the coating.

No agitation of the solution is required when a heated solution is used, because action is quite vigorous when the solution is hot. However, in order to insure that all portions of the work are exposed to the stripping solution, it is preferred to move the work through the solution so that stripping liquid flows relative to the coated surface of the work.

One method of operation found suitable has been to heat the solution to between about and F. before immersing the part to be stripped and then heating the stripping liquid to between about 185 and 200 F., as the stripping proceeds toward completion.

If the solution is permitted to cool to about 180 F. and no action is apparent, then the stripping is complete and the part is removed for rinsing in water.

Any smut which may be present after stripping is removed by liquid honing (vapor blast) and if removal of the coating does not appear to be complete, the part can be immersed in the stripping solution for a short time.

Another method found suitable has been to place the parts in the solution, heat to 165 F., hold at 165 F. for 2 hours, then remove from solution, rinse in water and proceed with liquid honing.

Usually immersion for between 30 minutes and 90 minutes is sufficient, but immersion overnight or for as long as 20 hours has not produced any attack or intergranular corrosion of the part being treated.

The method of the present invention has been applied successfully to salvaging engine-run parts, such as buckets or blades on which the aluminide coating has been eroded in service at only certain locations such as the leading edge.

After completion of the removal of the aluminide coating, the part is dried and is then ready for recoating according to the pack cementation procedure described in US. Pat. application Ser. No. 686,852 filed Nov. 30*, 1967, or other suitable process.

it should be particularly noted that the solution dissolves only the aluminide coating and does not attack the superalloy base material. It is also effective for the removal of nickel aluminide deposited by flame spraying or plasma arc spraying onto other metal substrates.

in some cases it may be desirable to strip only the excess coating to restore the part to specifications, thus saving the cost of completely stripping and recoating to specification.

The following data shows actual rates of stripping aluminide coatings at 165 F.:

Resultant Time in coating Coating strip, thickness, removal, Initial coating thickness, inches minutes inches inches TABLE Phosphoric acid 83... 80 G7... 50., 34..." 10 i0. Nitricucid .17.,.. 10. 25 33. 80 10. Acetic acid 0 0.. 33". 10 80. Water content 17.5... 15 20..... 15..." 15 25.6 4.5. Strip ratc Slow.. Moderate Fast Fast. Fast" Very fast Very slow.

1 Percent by volume. 2 Percent by woightcalculat ed.

Having now described the invention, it is not intended that it be limited except as required by the appended claims.

1. A process for removing aluminide coatings from substrates selected from the group consisting of Ni-base alloys Co-base alloys and superalloys comprising: immersing the coated substrate in a mixture of phosphoric, nitric and acetic acids, the duration of the immersion being for a time sufficient for the aluminide coating to dissolve.

2. The process of claim 1 wherein at least some of the acetic acid is replaced by chloroacetic acid.

3. The process of claim 1 wherein the immersion is in heated mixture of acids, heated to a temperature between about 130 F. and about 220 F.

4. The process of claim 1 wherein the mixture of acids is heated to between about 140 and 165 F. before an article to be stripped is immersed therein and then heated to between about 185 and 200 F. after the part is immersed and the stripping proceeds.

5. The process of claim wherein the stripped substrate is permitted to cool in the solution until no further action is visible.

6. The process of claim 1 wherein, after completion of stripping, the substrate is removed from the stripping mixture and is then liquid honed.

7. The process of claim 1 wherein the coating removed is nickel aluminide deposited by flame spraying.

8. The process of claim 1 where in the coating removed is nickel aluminide deposited by pack cementation.

9. The process of claim 1 wherein the coating removed is an aluminide formed by any means.

10. The process of claim 1 wherein the relative proportions of acids in the stripping mixtures are respectively 2:121 by volume, based on a phosphoric acid concentration of percent, nitric acid concentration of 70 percent, and acetic acid concentration of 99 percent.

11. The process of claim 10 wherein the temperature is about F.

PO-1O5Q UNITED STATES PATENT OFFICE 5G9 CERTIFICATE OF CORRECTION Patent No. 3,607,398 Dated September 1971 Inventor(s) Joseph Lucas It is certified that error appears in the above-identified patent and that said Letters Patent are hereby corrected as shown below:

Column 1, line 9, "be" should read by Column 1, line 50, change "015-85 to 0-85 Signed and sealed this 29th day of February 1972.

(SEAL) Attest:

ROBERT GOTTSGHALK EDWARD M.FLETCHER, JR.

Commissioner of Patents Attesting Officer 

2. The process of claim 1 wherein at least some of the acetic acid is replaced by chloroacetic acid.
 3. The process of claim 1 wherein the immersion is in heated mixture of acids, heated to a temperature between about 130* F. and about 220 F.
 4. The process of claim 1 wherein the mixture of acids is heated to between about 140* and 165* F. before an article to be stripped is immersed therein and then heated to between about 185* and 200 F. after the part is immersed and the stripping proceeds.
 5. The process of claim 1 wherein the stripped substrate is permitted to cool in the solution until no further action is visible.
 6. The process of claim 1 wherein, after completion of stripping, the substrate is removed from the stripping mixture and is then liquid honed.
 7. The process of claim 1 wherein the coating removed is nickel aluminide deposited by flame spraying.
 8. The process of claim 1 where in the coating removed is nickel aluminide deposited by pack cementation.
 9. The process of claim 1 wherein the coating removed is an aluminide formed by any means.
 10. The process of claim 1 wherein the relative proportions of acids in the stripping mixtures are respectively 2:1:1 by volume, based on a phosphoric acid concentration of 85 percent, nitric acid concentration of 70 percent, and acetic acid concentration of 99 percent.
 11. The process of claim 10 wherein the temperature is about 165* F. 